Compared to a HDD (Hard Disk Drive) or other such storage medium, a flash memory is characterized by the fact that it degrades significantly as the number of writes increases. A storage apparatus, which makes use of flash memory, is generally controlled so as to extend the life of the storage apparatus as long as possible even when using the flash memory by restricting the number of writes, restricting the write data size, and deleting data on a regular basis. For example, in a certain type of storage apparatus, overwriting data to a physical storage area is prohibited, and in a case where a write request for updating data has been received, control is exercised such that a new physical storage area is allocated to the request-destination logical storage area, the update data is stored in the new physical storage area, and the physical storage area in which the pre-update old data is stored is treated as an invalid area and made inaccessible from the outside.
Meanwhile, as a backup function, the storage system has a snap shot function, which stores data of a certain point in time in the past and provides a past data image.
There has been proposed a method (Patent Literature 1) in which a storage system that uses a flash memory as a storage medium utilizes the above-mentioned control by which pre-update old data is stored in the system even after update data has been stored, manages this pre-update old data as data for use in a snap shot, which maintains the association between the physical storage area in which the old data is stored and the logical storage area, and provides a past data image by using the relevant data and data that has been stored in another logical storage area with respect to which an update data write request has not been issued.